1. Field of the Invention
The invention refers to a device for flame stabilization in a burner, with a burner housing at least partially enclosing a burner volume into which may be introduced via at least one fuel line, fuel, and via at least one air feed means, air, forming an air/fuel mixture spreading in a preferred flow direction, which air/fuel mixture may be ignited in a combustion chamber connecting downstream of the burner housing to form a stationary flame. In addition, a method for flame stabilization in a burner related to this is described.
2. Brief Description of the Related Art
Modern premix burners, as a representative of which example reference is made to a premix burner with a conical burner housing, which is described in EP 321 809 B1, are optimized from the point of view of their efficiency as well as with regard to their pollutant emissions. The optimizations carried out on the burner systems are valid especially for load ranges in which such burner systems are mainly operated in order to drive, for example, heat engines, mainly gas- or steam turbine installations. Such installations are operated for most of the time under full- or partial load conditions.
From the aforementioned example of a conically constructed premix burner, attention should be drawn subsequently to a problem which arises during the operation of such burners. The embodiments mentioned below are not necessarily limited to conical premix burners. On the contrary, the problem relates to all generic premix burners.
In a manner known per se, modern premix burners include conically widening burner volumes, the so-called swirl chamber, into which air and fuel are fed forming a swirled flow conically widening axially in the direction of the swirl chamber. By the provision of an inconstant flow transition between the swirl chamber and the combustion chamber housing connecting to the swirl chamber, the swirled flow splits and forms inside the combustion chamber a reverse flow zone in which the fuel mixture ignites forming a spatially largely stationary flame. In order to be able to ensure a combustion process which is as optimized as possible, it is necessary to promote flame development which is as homogenous and spatially stationary as possible.
Such burners are, however, unavoidable if operated even only temporarily under load- and operating conditions, under which a homogenously developing, spatially stationary flame cannot be formed or can be formed only with considerable limitations. Especially under start- and low load conditions, corresponding measures for flame stabilization have to be taken to ensure the demands made for the flame quality. A tried and tested apparatus for flame stabilization constitutes the so-called pilot gas feed by which the added pilot gas which experiences no premixing or only slight premixing with the feed air is fed to the flame mostly via a burner lance installed centrally in the burner. Such pilot gas feeds lead to so-called pilot flames which are basically of the diffusion type, even in cases in which the premix burner is operated under lean fuel conditions.
A further measure for flame stabilization provides for the use of catalysts which, within the scope of a so-called catalytic piloting, are provided in the mixing region of a premix burner, and, depending on the air/fuel ratio λ and also on the oxygen present in the mixture, oxidize at least portions of the fuel contained in the air/fuel mixture. It is possible, by use of catalytic reactors inside the premix burner region, to produce by partial oxidation of the fuel portion so-called syngas which consists of H2 and CO and, on the basis of the hydrogen content, constitutes a highly reactive gas, especially in the case of a rich air/fuel mixture, i.e., λ<1. In this way it was able to be experimentally proved that a specific admixing of syngas into the flame region developing in the combustion chamber, an improved combustion stability with regard to a stable flame position, and also a reduced nitrogen monoxide emission can be achieved (see Samuelsen, 99-GT-359, ASMA-Turbo Indianapolis).
It is also known to create, by catalytic partial oxidation, an air/fuel mixture developing inside a burner, and to create, by suitable selection of the air/fuel ratio and inlet temperatures of the air/fuel mixture in the catalytic reactor, a syngas-free gas mixture consisting of CH4, N2, CO2, and H2O which, on account of the methane contained in the gas mixture, corresponds to a conventional, lean, premixed pilot gas. Such a method is to be gathered from U.S. Pat. No. 6,358,040 and also U.S. Pat. No. 6,394,791, for example. A method can be taken in each case from these publications in which the air/fuel mixture partially oxidized by way of catalysis is mixed with cooling air in order to avoid spontaneous ignitions and a diffusion flame connected with it and to be ultimately fed as a hot, lean, CH4-containing mixture for the purpose of the stabilization of the flame homogenously developing inside the combustion chamber.
All three previously described measures, be it the feed of pilot gas forming a diffusion flame or the use of catalytic reactors for producing syngas-containing or syngas-free, but in any case CH4-containing, gas mixtures, are based on the mixing of a hot, reactive pilot gas with the air/fuel mixture developing in the premix burner. In all cases it is consequently crucial that a complete mixing of the reactive pilot gas with the air/fuel mixture is produced before spontaneous ignitions occur in order to ultimately avoid so-called hotspots and also increased nitrogen oxide emissions. By the additional feed of a reactive pilot gas, the flame position, moreover, can change inside the combustion chamber, which causes a reduction in the time span of the complete mixture formation, especially in that case in which the flame assumes a combustion chamber-internal upstream orientated position. Obviously, an increased formation and emission of nitrogen oxides is associated therewith.
The influence on the spatial position of the homogenous flame developing inside the combustion chamber is, by means of a pilot gas feed, greater the richer in fuel the supplied pilot gas is. The place of the feed of syngas relative to the flame position is of significant importance, in particular during the possible syngas formation by way of the catalytically promoted partial oxidation, especially since the flame position could react very sensitively with regard to a syngas feed. These dependencies of the flame position associated with syngas feed are explained in detail in U.S. Pat. No. 5,937,632 and described within the scope of a so-called chemical flame stabilization.
To sum up, it can consequently be emphasized that problems face the previously described measures for flame stabilization during the operation of modern premix burners, especially under partial load conditions or during the starting phase.
It is necessary on the one hand to avoid the formation of so-called hot pockets, i.e., unburnt fuel, which reacts with the air/fuel mixture of the main flow before the mixture has experienced complete mixing. On the other hand, the piloting technique previously in use influences the flame position and thus the available time for the complete mixing of the air/fuel mixture which with premature ignition releases a considerable nitrogen oxide portion.